lec 15.docx

IMAGE FORMATION BY THE EYE
Cornea, rather than the lens, is the site of most of the refractive power of the eyes
Refraction by the Cornea
Light strikes the cornea and passes from the air into the aqueous humor. Then light rays bend
and converge on the back of the eye (see Fig. 9.7)
 Focal distance: the distance from the refractive surface to the point where parallel light rays
converge and depends on the curvature of the cornea (i.e. tight the curve, the shorter the focal
distance).
 Diopter: reciprocal of the focal distance in meters. The cornea has a refractive power of 42
diopters which means that parallel light rays striking the corneal surface will be focused 0.024 m
(2.4 cm).
Refractive power depends on the slowing of light at the air-cornea interface. If we replace air
with a medium that passes light at about the same speed as the eye, the refractive power will
eliminated. This is why one’s vision is blurry when you open your eyes underwater.
Accommodation by the Lens
The lens contributes another dozen or so diopters to the formation of a sharp image at a
distance. It is, however, more importantly involved in forming crisp images of objects located
closer than about 9m from the eye.
 As objects approach, the light rays originating at a point can no longer be consider to be parallel.
These rays diverge and greater refractive power is requir